The cardiac effects of high dose (180 mg/kg) cyclophosphamide (CTX) were studied in 14 patients by serial echocardiograms and ECG. The ECG's of an additional 17 patients were reviewed. Summated QRS voltage (Sigma v) decreased in 27/31 (81%) patients with maximum decline 2-14 days (mean 7 days) after beginning CTX. Fractional shortening decreased in 10/3 patients (77%). Although pericardial effusion occurred in 9/31 patients (29%), CTX was associated in with decreased Sigma v in 8 patients in whom effusion was excluded by echo. Congestive heart failure occurred in 12/32 (39%) patients resulting in 7 deaths (23%) 6-14 (mean 10 days) after starting CTX. Morbidity and mortality from cardiac toxicity of this regimen appear to be high.